On Monday, New Delhi woke up under a dense blanket of grey haze, with its air quality remaining stubbornly in the “very poor” category. According to data from the Central Pollution Control Board (CPCB), the city’s overall Air Quality Index (AQI) registered at 316, and 28 monitoring stations reported values above 300.
Some locales were far worse - for example, one station recorded a reading of 561, amounting to hazardous levels. 
Experts attribute the spike to stagnant winds, cooler temperatures that trap pollutants near the ground, vehicle emissions, construction dust and smoke from nearby crop-burning states. Authorities have warned that the situation is likely to deteriorate further, with the city possibly entering the “severe” pollution category later this week.
Residents - especially children, the elderly and those with lung or heart issues - have been advised to limit outdoor activity, wear masks and stay indoors where possible.